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You may want to check with the machinists instead of the guest muppet, but I think thinner angle or better yet box section would be a lot better for bracing than flat bar if your design will allow it, flat will flex too much?

I know others have chimed in and given very good information, my two cents is as follows:

Originally Posted by JoeHarris

Further to my previous post I have some questions:

1) to drive the two ball screws on the x axis I am using a belt driven by one stepper and driver. I am doing this because of fear of missed steps. If I use digital drivers (not closed loop) does this issue go away?No digital driver do not remove the risk of missed steps. I am running twin screws with twin drives (one slaved to the other via Mach) and using digital drives (the very AM882's that Jazz refers to). I had been running them at 8.5m/min velocity and 1.75m/sec^2 acceleration for a good while, without any stalling what so ever at all. That was for the X axis as each one I tune a bit different. I am using 1605 ballscrews and direct drive. Currently I am running them at 8m/min V and 1.5m/sec^2 A and the little bit of jerkiness that was present has gone. Running a good solid voltage for the motors helps a lot. Currently running 60V and will be running 68V when I finish building the new power supply (almost have al the parts and slowly getting comfortable with how to d o it correctly).

2) if I stick with the belt drive arrangement shown does it matter where the motor is placed in the belt loop. I have shown it off to one side currently, I had it in the middle previously? I figure it shouldn't make a difference? Probably need to add in another idler though.

I will leave this to those who have experience with belt drive systems.

3) the four sides of the base of the machine is comprised of two aluminium plates fixed to the ends of two aluminium profiles. These sit onto a series of aluminium box sections. I have no cross bracing in this frame. Would it be prudent to add some in??

Yes, rigidity is your friend as long as it does NOT cause binding or increase enertia unduly.

I intend to build the timber table next so I have a base to work up from - any comment would be greatly appreciated before it is too late!!
Yes when building your table do the best you can to make sure the surface is truly flat and set up points with which to bolt the machine to it so that once built the timber frame can be secured and act as additional part of the rigid structure to help absorb vibration from the machine.

The more I think about how to brace this thing up I am moving towards a metal frame rather than timber. I would keep all of the aluminium profile and plate as is but bolt this down to a steel box frame. The question is, is it worth buying an arc welder and learning to weld for the gains in strength and hopefully accuracy??

Learning to weld is a skill thats worthwhile in itself. The thing about welding up a steel frame is that unless you are careful it will distort with the heat. The mistake that most beginners make (and I include myself in that as I'm not an expert welder, I just get by) is to try and weld too much in one go. The trick is to tack things in place then go back and fill the gaps so to speak. You dont need to seam weld everything. Personally I'd go with a MIG welder rather than an arc/stick welder. Its easier to strike the arc and get a good finish. While a steel box won't necessarily improve accuracy it will improve rigidity and you can still use wood with steel; a basic 4-sided bed with 4 legs of steel and a carcass of mdf bolted over will give it rigidity (and turn the base into a useful cabinet)

Like irving has said welding is a useful skill to have but like anything does take a bit of practice to get right. Mig is certainly easier to learn than stick & probably more forgiving in that it is easier to keep a run going. Steel is pretty cheap to buy so the practice doesn't really cost a lot. If you picked up a decent second hand mig machine you would probably be able to sell it on if you decided not to keep it for much the same as you paid for it so would only really be borrowing it for your build.
Even with buying a welder a steel frame will probably work out cheaper than an aluminium frame I would have thought. Can't speak from a cnc point of view though as haven't built a machine yet myself lol.

I came to pretty much the same conclusion about using steel, I can upgrade something like a spindle or motor at a later fairly easily to improve the machine but if the frame moves around like a jelly then the whole thing would have to go in the bin and get rebuilt from scratch.

At the moment I'm planning to hold thep pieces of steel together with brackets made from steel angle and bolts so that I can adjust it fairly easily, because of the problems welding seems to cause I'm also considering joining some bits with a metal epoxy. If you need to buy a welder just for this job (the really cheap mig welders won't do 3mm/4mm box properly) the epoxy route would work out cheaper and easier than learning how how to weld on a frame that needs precision work from the get go.

This guy has a lot of useful info for how to approach frame building, while spending £500 on a perfect straight edge to use for alignment may be out of my league there is some good stuff that can be applied on a lower budget.

Meant to say if you buy a gas less mig setup you don't have to worry about insurance as far as having gas bottles on the premises goes either. Not used gasless mig myself but know people that do & seems to be little difference.

That's an interesting read DC, haven't read it all yet but did read the bit about him using an epoxy to basically glue the steel sections together.
Have you done much research on this way of manufacturing a frame? The guy doesn't seem to go into to much detail & I couldn't see much on the link he provided but haven't read that properly either.
He mentioned using west systems 207 which is just a hardener so not sure what resin he used plus there is no mention of ratios for resin, hardener or fillers although there would have to be a low volume of fillers in it if you can pour it.